1. Field of the Invention
This invention relates to an in-line type electron gun used in a high resolution color cathode ray tube, and more particularly to an in-line type electron gun enhanced in performance by unifying the focus voltage and beam spot shape of three electron beams.
2. Description of the Prior Art
FIG. 1 shows the structure of a conventional in-line type electron gun, in its side section (a) and front section (b) by line A-B.
In the figure, numeral 1 is a cathode, 2 is a number 1 grid (G1 grid) disposd before the cathode 1, 3 is a number 2 grid (G2 grid) disposed before the G1 grid 2, 4 is a number 3 grid (G3 grid) disposed before the G2 grid 3, and 5 is a number 4 grid (G4 grid) disposed before the G3 grid 4.
A triode is composed of G1 grid 2 and G2 grid 3. A main focusing lens is formed by G3 grid 4 and G4 grid 5 between them (the position of line A-B in the drawing), in which the G3 grid 4 is the lower voltage side grid. Numeral 6 is a beed glass for fixing the grids 2, 3, 4, 5. Numeral 7 is a depression provided in the front face of the G3 grid 4, more specifically, in the face facing the opposing gap to the G4 grid 5. As a voltage is applied between G3 grid 4 and G4 grid 5, a magnetic field is generated, and the main focusing lens common to three electron beams is formed in the depression 7. At the bottom of the depression 7, there are three lens holes 71, 72, 73 of identical diameter provided in-line so as to form small lenses for individual electron beams corresponding to the red, green and blue colors.
By disposing the depression 7 in front of confronting side of the G3 grid 4 and G4 grid 5, the structure to form a common main focusing lens in the position of the depression 7 aside from the small lenses for individual electron beams formed in the lens holes 71 to 73 is disclosed by A. M. Morrel as "An Overview of the COTY-29 Tube System: An Improved Generation of Color Tubes" in IEEE Transaction on Consumer, Vol. CE-28, No. 3, August 1982.
In this in-line type electron gun, the electron beam gaps may be narrowed with almost no influence to the focus characteristics.
In this disclosure, meanwhile, the depression 7 is oval, more specifically, formed in a form of a race track oblong in the direction of lens hole array, and the diameters of three lens holes 71 to 73 are all identical.
In such conventional in-line type electron gun, the thermion generated by the cathode 1 is pulled out and accelerated by voltage applied to the G1 grid 2 and G2 grid 3 which make up a triode. And the electron field caused by the difference in voltage applied to the G3 grid 4 and G4 grid 5 (as mentioned above, the voltage is lower at the G3 grid 4 side) will form a main focusing lens in the depression 7, which causes to warp the orbit of the electron, so that the three electron beams are converged to be concentrated on one point at the position before the G4 grid 5.
In this in-line type electron gun, the three lens holes (beam passing holes) 71 to 73 provided at the bottom of the depression 7 in front of the G3 grid 4 in which the main focusing lens is formed are made in identical diameter. Therefore, by the three-dimensional effect of the electric field, the focus voltage differs between the two outer lens holes 71, 73 and the center hole 72. As a result, the shapes of spots of the beam from the center lens hole 72, and of the beams from the two outer lens holes 71, 73 (that is, the shadow projected on the intersecting plane of the beam running directions) are different, and sharpness of the picture is lowered.